North Korea has issued another belligerent threat, warning of an indiscriminate "pre-emptive nuclear strike of justice" on Washington and Seoul, following the start of huge US-South Korean military drills.

North Korea has issued another belligerent threat, warning of an indiscriminate "pre-emptive nuclear strike of justice" on Washington and Seoul, following the start of huge US-South Korean military drills.

Such threats have been a staple of North Korean leader Kim Jong Un since he took power after his dictator father's death in December 2011. But they increase especially when Washington and Seoul stage what they call annual defensive spring war games.

Pyongyang says the drills, which started on Monday and continue through the end of April, are invasion rehearsals.

The North's powerful National Defence Commission threatened strikes against targets in the South, US bases in the Pacific and America's mainland, saying its enemies "are working with bloodshot eyes to infringe upon the dignity, sovereignty and vital rights" of North Korea.

"If we push the buttons to annihilate the enemies even right now, all bases of provocations will be reduced to seas in flames and ashes in a moment," the North's statement said.

South Korean Defence Ministry spokesman Moon Sang Gyun said North Korea must refrain from a "rash act that brings destruction upon itself".

In Washington, State Department spokesman John Kirby said the US took the North Korean threats seriously and called on Pyongyang to cease provocative rhetoric and behaviour.

"There would not be as compelling a reason to prepare for alliance capabilities if Pyongyang wasn't so intent on raising the stakes on the peninsula" and decreasing any sense of security and stability there, he said.

This year's war games will be the largest staged, involving 300,000 South Korean and 17,000 US troops.

South Korea's Yonhap news agency, citing military sources, said the allies would work on drills for precision attacks on the North Korean leadership and its nuclear and missile arsenal in the event of war.

A pre-emptive large-scale military strike that would end the authoritarian rule of the Kim dynasty is highly unlikely. There is also considerable outside debate about whether North Korea is even capable of the kind of "strikes" it threatens.

The North makes progress with each new nuclear test - it staged its fourth in January - but many experts say its arsenal may consist only of still-crude nuclear bombs.

There is uncertainty about whether it has mastered the miniaturisation process needed to mount bombs on long-range missiles and widespread doubt about whether it has a reliable missile that could deliver such a bomb to the US mainland.

But North Korea's bellicose rhetoric raises unease in Seoul and its US ally, not least because of the huge number of troops and weaponry facing off along the world's most heavily armed border, an hour's drive from the South Korean capital and its 10 million residents.

The rival Koreas' usual animosity occasionally erupts in bloody skirmishes - 50 South Koreans were killed in attacks in 2010 that Seoul blames on the North - and there is always a worry about an escalation of violence.

Always ragged relations between North Korea and its rivals Seoul and Washington have worsened following North Korea's nuclear test in January and a long-range rocket test last month that outsiders say was a test of banned ballistic missile technology.

The United Nations recently slapped the North with harsh sanctions and South Korea has taken a harder than usual line, with a new North Korean human rights law and the president in Seoul warning of a collapsed government in Pyongyang.

Similar nuclear threats by the North were made in 2013, around the time of the spring-time military drills, after the UN sanctioned the North over a nuclear test and long-range rocket launch.

Analysts say one part of North Korea's traditional anger over the drills is that they force the impoverished country to respond with its own costly war games.

Meanwhile, South Korea said it would sanction 40 individuals and 30 organisations aboard, mostly in North Korea, as part of its unilateral punitive measures against Pyongyang for its recent nuclear test and rocket launch.

The South Korean government also said it would ban the entrance of any ship which has also stopped at a North Korean port in the previous 180 days. Currently, only North Korean ships are banned.

The sanctions target 38 individuals and 24 organisations in North Korea, and two individuals and six organisations in other countries.